1. Field of the Invention
This invention relates to methods and apparatus for dewatering quenched slag, such as slag discharged from the lockhoppers or other systems for dumping slag which has been quenched in water after generation in coal gasification systems or the like.
2. Information Disclosure Statement
Due to political and economic events, the United States has endured several energy crises in recent history. Some industries have found it expedient to utilize equipment which allows the use of alternate fuels, e.g., petroleum fuel oils or natural gas, according to availability or economics. Historically, the most available and economical source of motor fuels has been the refining of petroleum, although nations such as Nazi Germany and South Africa have found it necessary to employ sources of synthetic fuels such as coal from time to time. As economically available domestic and foreign sources of petroleum and natural gas dwindle, the strategic and economic value of America's vast coal reserves increases. In addition to the logistic and economic problems of recovering and transporting such resources to the point of use, the problems of coal use include atmospheric emissions and the disposition of large volumes of ashes and slag, which may allow heavy metals and other pollutants to be leached out by water.
In a typical coal gasification plant, a particulate fuel slurry containing coal is delivered to a gasifier where it is combined with oxygen to form a synthesis or fuel gas ("syngas") by a partial oxidation reaction. The operating temperature is generally maintained sufficiently above the ash fusion temperature to ensure that the slag generated is in a free-flowing molten state. The hot crude syngas generated is subjected to high temperature gas cooling or quenching when it is introduced, together with the molten slag, into radiant syngas coolers or quench sump(s).
Typical radiant coolers are disclosed in coassigned U.S. Pat. Nos. 2,896,927 and 2,818,326, which are incorporated herein by reference. These coolers generate high pressure steam and a cleaned gas which is directed to convective coolers. In the "direct quench" type plant, as described, e.g., in U.S. Pat. No. 4,052,176 (col. 4), the hot crude syngas is directly quenched in water. The molten slag drops to the bottom of the radiant coolers where it is quenched in a water pool (common to both gasifier configurations), forming a coarse slag which settles at the bottom of the sumps. The quenched slag is periodically discharged through a water-filled lockhopper system. Typically, the slag-water mixture discharged from the lockhoppers is discharged to a slag sump tank where the slag is allowed to settle in the accumulated water. The settled slag is then removed by drag conveyor or similar transport systems to a concrete-lined area for truck loading and the clarified water is pumped to filters which remove fine particulate matter (fines). Although such systems are operational, they have numerous disadvantages; for example, reliable and sturdy drag conveyor systems are expensive and require frequent maintenance and repair due to the large volumes of heavy, abrasive and corrosive slag handled. The typical flat-bottomed slag sump tank is difficult to empty of slag liquids and accumulated fines, and periodic shutdowns may be required if only one sump tank system is available for dumping of slag from the lockhoppers.
Inadequately drained slag can have up to 60 to 70 weight percent water. Thoroughly drained slag contains about 25 percent water and appears dry. Even after 30 days in the dry desert atmosphere slag can contain as much as 14 percent water. When inadequately drained slag is loaded into trucks water can drain out during loading and transport to the eventual disposal site. Such drainage of water with accompanying fines onto the loading site and the roadways increases the problems of site clean-up and reduces the amount of dry slag that the truck can transport per trip because of the added weight of unwanted water. Furthermore, when the wet slag is loaded into a truck the seepage of water interferes with the seating of the dumpster or tailgate surfaces because the fines prevent a tight fit. Unless the equipment is thoroughly hosed down, the improper seating of the mating surfaces aggravates spillage during transport.
Clearly, improved methods of dewatering, transporting and disposing of quenched slag and similar particulate materials containing excess aqueous liquids are needed.